1. Field of the Invention
The present invention relates to a flux-cored wire for gas-shielded arc welding to be used for welding materials formed of heat-resisting low alloy steels and used in various plants, such as nuclear power plants, thermal power plants and petroleum refining plants. More particularly, the present invention relates to a flux-cored wire for gas-shielded arc welding for welding materials formed of heat-resisting low alloy steels, capable of suppressing or completely preventing the formation of ferrite bands in weld metals when the weld metals are subjected to a post weld heat treatment (hereinafter, abbreviated to “PWHT”) at high temperatures for a long time, of forming weld metals having high tensile strength and high toughness, and of facilitating welding work.
2. Description of the Related Art
Welding wires for gas-shielded arc welding are classified into solid wires and flux-cored wires. Flux-cored wires, as compared with solid wires, have various advantages including capabilities to cause less spattering, to form beads in satisfactory appearance and shape, and to facilitate vertical-position welding and overhead-position welding as well as flat-position welding. Accordingly, the use of flux-cored wires for welding materials formed of heat-resisting low alloy steels have progressively been increased.
Since welded structures formed by welding together materials formed of heat-resisting low alloy steels are used in high-temperature, high-pressure environments, flux-cored wires to be used for constructing such welded structures are required to have characteristics meeting working conditions required by such working environments. Generally, weld joints in materials formed of heat-resisting low alloy steels are subjected to a PWHT to reduce residual stress, to remove residual hydrogen and to improve mechanical property. Therefore, flux-cored wires for gas-shielded arc welding for welding materials formed of heat-resisting low alloy steels are required to be capable of preventing the deterioration of the characteristics of weld metals.
When a conventional flux-cored wire is used for the gas-shielded arc welding of materials formed of heat-resisting low alloy steels, ferrite bands are formed in weld metals and the mechanical properties of the weld metals are deteriorated when the weld joints are subjected to a PWHT of high temperatures and a long duration. More concretely, ferrite bands reduce the tensile strength of weld metals. It is considered that ferrite band is caused by the segregation of the component metals during the solidification of the weld metals and the migration of carbon contained in the weld metals during the PWHT.
Some techniques have been proposed to solve such problems. A technique disclosed in JP-B No. 8-13432 (hereinafter referred to “Reference 1”) adds both Nb and V, which are elements having high ability to form carbides, to a flux-cored wire to suppress ferrite band through the suppression of the migration of carbon in weld metals. The inventors of the present invention proposed previously a technique in JP-A No. 2001-314996 (hereinafter referred to as “Reference 2”) for suppressing ferrite band by properly adjusting the TiO2, alkaline metal compound and fluoride contents of a titania flux-cored wire on the basis of knowledge that the improvement of the stability of arcs reduces the segregation of the components of alloys forming the weld metals and the resulting ferrite band and improves the mechanical properties of the weld metals. A technique for improving the toughness of weld metals that adds N in wires in a proper N content is disclosed in JP-A No. 57-4397 and JP-B No. 62-19959 (hereinafter referred to as “Reference 3”). A zirconia flux-cored wire proposed in JP-B Nos. 2-42313 and 3-3558 (hereinafter referred to as “Reference 4”) reduces the TiO2 content of the flux, i.e., a source of Ti, and the Ti content of the wire to the least possible extent by prescribing TiO2 content and metal Ti content under predetermined limiting conditions or by prescribing TiO2 content and metal Ti content under predetermined limiting conditions and limiting N content to a proper range on the basis of knowledge that Ti contained in the weld metals reduces the toughness of the weld metals.
Although the technique disclosed in Reference 1 that adds Nb and V to a flux-cored wire is effective in suppressing the migration of carbon to prevent ferrite band. Nb and V are elements that reduce the toughness of weld metals greatly. Thus, Reference 1 is unsatisfactory in ensuring that weld metals have sufficient toughness. Reference 2 has difficulty in meeting demand for higher toughness. Reference 3 does not disclose any technique for preventing ferrite band and does not disclose any technique for ensuring that weld metals have satisfactory toughness, and preventing ferrite band. The zirconia flux-cored wire disclosed in Reference 4 is inferior to titania flux-cored wires in wire usability in a vertical position and an overhead position.